Embodiments of the present disclosure relate to an impedance calibration circuit, and more particularly to a technology for improving precision of pad resistance.
A termination resistor having the same resistance as characteristic impedance of a transmission (Tx) channel is coupled to a receiver or a transmitter of a semiconductor device. The termination resistor matches impedance of the receiver or the transmitter to characteristic impedance of the transmission (Tx channel, such that it can suppress reflection of signals transmitted through the Tx channel. Although termination resistors according to the related art have been installed outside the semiconductor device, the latest termination resistors have been implemented as On Die Termination (ODT) circuits in which termination resistors are installed inside the semiconductor device.
Each ODT circuit includes a switching circuit that is turned on or off to control a current flowing in the semiconductor device, such that power consumption of the ODT circuit is less than that of an external termination resistor installed outside. Since a resistance value of the ODT circuit is changed according to PVT (Process, Voltage, Temperature) conditions, the resistance value of the ODT circuit is calibrated using an impedance calibration (ZQ calibration) circuit, prior to using the ODT circuit.
The impedance calibration (ZQ calibration) circuit includes a comparator configured to compare a resistance value of an external resistor connected to a pad with a reference voltage, such that a pull-up code and a pull-down code for calibrating the resistance value of the ODT circuit are generated. In this case, the external resistor connected to the pad may have a constant resistance value (generally, 240Ω) irrespective of the PVT condition change.